Also, if you have a connection with Wella, they have angled freelights brushes which are great for softer lines. I would do the four sections, roughly paint on your root colour and comb with a wide tooth comb as you go. After applying the roots, I would go through with the angled brush (or side of a regular tint brush) and brush through again giving the roots a deeper colour and then the faded mid section will be slightly lighter. I also like to emulsify down a cm or 2 when rinsing and leave it for just a minute or two to try and soften the lines a bit more.